In-time detection of water pollution by oil remains a relevant task to prevent environmental damage due to the spill and pollution spreading and loss of the product caused by the leak incidents. Among the variety of the techniques used for the task, the most demanded are the ones capable of operating in real-time, unattended, and with minimal maintenance.
The Light Induced Fluorescence (LIF) is well known high sensitive method used for detection of oil in water. It is referred to as an active method due to the use of controlled light source. The selectivity of oil detection with this method is provided due to the fact that at properly selected spectrum of the inducing light the poly-aromatic hydrocarbons (PAH) of oil products produce fluorescence with specific spectral profile. This profile differs from spectral response of other materials to the same inducing light and due to that can be used for oil detection and quantification.
Another feature of the LIF method is that it can be applied remotely without sampling or contacting the water body. This feature in combination with high selectivity, sensitivity and instantaneous detection allows building the sensor based on LIF method operating continuously in non-contact and real-time mode.
LIF method is effectively realized for oil detection by airborne and shipboard LIDAR (Light Detection and Ranging) monitoring systems. These systems provide high sensitivity and spatial resolution down to meter scale and are capable to detect oil on the water surface, submerged oil, and dissolved and emulsified fractions. Being very effective as mobile units, such systems are often too complex and expensive for continuous local on-site monitoring.
The remote (non-contact) mode of oil detection with LIF method is realized by fluorescence LIDARs (Light Detection and Ranging). The invention according to patent document U.S. Pat. No. 5,096,293 “Differential fluorescence LIDAR and associated detection method” to Cecchi Giovanna, Pantani Luca, 1992 discloses the use of fluorescence LIDAR for real-time remote sensing, and in particular for remote sensing of water. The detection method is based on the inducing the emission of fluorescence radiation from a remote target and collecting such emission with following separation into plurality of predetermined spectral channels. Collected spectral signals are processed in a way of calculating the ratio of each channel signal to each other signal thus providing a form of ratios which is compared further with predetermined stored signals corresponding to defined targets or categories of targets. The method is realized by the system comprised the pulsed laser with optical means to direct the laser beam to the object and to collect back scattered radiation; optical channel separator means for separating a backscattered fluorescence signal into predefined number of channels, and signal processing means for forming a plurality of ratio signals and comparing the value of each ratio signal to values stored in a memory.
In the patent document WO2012015332 “Method for remote detection of oil pollution on the surface of water”, to Belov, Mikhail Leonidovich et al, 2012 there is described a method for oil detection by irradiating the surface of the water in the ultraviolet range at the excitation wavelength and recording the intensity of the fluorescent radiation in two narrow spectral ranges with centers at wavelengths λ1, λ2. The wavelengths are selected from the condition for the maximum difference between the value of the radiation intensity ratio (for petroleum products) and the value of the radiation intensity ratio (for water). The measured value for the radiation intensity ratio is compared with threshold values.
The above method is based on the assumption that it is possible to define two wavelengths of fluorescence emission of oil with distinct ratio of intensities compared with that of pure water. Such an assumption does not work in case of oil products having the fluorescence in too distinguishing spectral ranges. For example, at the excitation wavelength 308 nm light oil (gasoline) has a max fluorescence at the emission wavelength 380 nm, and heavy oil (ship fuel) has a max fluorescence at the wavelength 520 nm. There is no any pair of wavelengths providing ratio of intensity distinguishing from clean water and equally usable for detection of both mentioned type of oils.
There is an invention defined in U.S. Pat. No. 7,688,428 “Non-contact oil spill detection apparatus and method” to Pearlman; Michael D, 2010 describing a non-contact sensor for oil spill detection based on LIF method.
The invention in U.S. Pat. No. 7,688,428 claims the oil detection method based on the anomalous signal return when oil is present compared with the signal when oil is absent. As the invention does not specify any means for taking into account ambient light and minimizing its influence, one can assume that the variation of ambient light due to glint reflection of sun light from the water surface could produce anomalous increase of the signal in the spectral range 320-400 nm and therefore the method will produce false alarm. As the method does not specify the data analysis, one can assume registering the anomalous signals caused by other than oil light reflecting materials on water surface, also possibly producing false alarms.
The sensor of U.S. Pat. No. 7,688,428 is based on the pulsed light source to produce a beam of light having the apex angle 13-15 degrees and spectrally limited to wavelengths between 225 and 300 nm, which is directed to the water; sensor means for detecting fluorescent light filtered in the spectral range between 320 to 400 nm, means for analyzing data and reporting and/or producing a signal or alarm when analyzed data indicates the presence of any significant amount of oil pollutant. The invention in U.S. Pat. No. 7,688,428 claims that high power strobed Xe lamp serves as a light source. The sensor is operated vertically above the water at the distance of 3 to 5 m from the surface and at the angle not more than 7 degrees from vertical. The invention also claims the method of oil detection predicated upon differential measurement, i.e. based on the anomalous signal return that will occur when oil is present, as opposed to when there are no hydrocarbons present.
The sensor described in U.S. Pat. No. 7,688,428 has the following limitations.
1) Using powerful Xe-lamp as a light source makes the sensor bulky and power consuming, causing principal limitations of the detection distance due to significant beam divergence (light beam has a shape of cone with apex angle 13-15 degrees), increase of the distance will require unreasonable increase of the power consumption.2) Inclination of the incidence beam of about 7 degree of vertical to detect sufficient amount of fluorescent flux at the detector indicates that the optical axes of the sensing light beam and detection means do not coincide. The signal detection with optical scheme having misaligned optical axes of the light beam and optical axis of receiving means (bi-static optical layout) is exposed to the distance variation. Therefore even if the sensor is adjusted for the distance to water (for example 3-5 m), the change of the distance due to tides, waves or other processes will cause the fluctuations of fluorescence signal thus influencing the reliability of detection.3) Due to above described reason the sensor cannot operate at the inclination angle more than 10 degrees from vertical. It limits its installation requirements and hampers operation in scanning mode when the inclination angle is varied.4) The detection limited to the spectral range 320-400 nm does not allow detection of oil products fluorescing at longer wavelengths (FIG. 2).